A wide variety of organic and inorganic compounds are used for the complex series of steps by which a negative-working silver halide emulsion of high sensitivity is produced. One such step involves the chemical sensitization of the silver halide grains to increase their light sensitivity.
During chemical sensitization, the individual grains of a negative silver halide emulsion undergo a localized surface reaction to produce sensitivity specks which are believed to consist of silver, gold, silver sulfide or some combination thereof.
Between the time of adding the sensitizer and coating the liquid emulsion on a support, the emulsion is usually given a heat treatment, called digestion. During digestion a reaction is believed to occur which produces sensitivity sites on the surface of the silver halide grains. Unfortunately, as the digestion reaction is continued in order to obtain a higher level of sensitivity, some silver halide grains become spontaneously developable without exposure. This causes the emulsion to fog. Films made with grains which have undergone digestion to achive high sensitivity not only exhibit this fog when tested shortly after being coated, but display higher levels of fog as the film is aged. This may reach a level such that the film is unusable and in any case limits the useful life of the film. Undesirable losses in sensitivity may also accompany the increase in fog as the film ages.
Efforts to obtain higher sensitivity for negative-working silver halide emulsions must in some fashion deal with the problem. One practical method of doing this is to tolerate some acceptable fog level in commercial photographic emulsions. Another is to add antifoggant or stabilizer compounds to reduce fresh fog and/or to prevent the formation of aging fog while accepting some sacrifice of sensitivity as a tradeoff for the improvement.
The present invention attacks the fog problem encountered in negative-working silver halide emulsions by providing a group of organic oxidants and radical traps which are selectively effective in eliminating fresh fog from highly sensitized emulsions, and in preventing formation of fog on aging.